Electrostatic fluidized bed coating is now a conventional and widely-used technique for depositing particulate materials upon a great diversity of workpieces. Methods and apparatus for electrostatic coating are well known in the art, as broadly exemplified by Knudsen U.S. Pat. Nos. 3,916,826 and 4,101,687, issued respectively on Nov. 4, 1975 and Jul. 18, 1978, and Karr U.S. Pat. No. 4,030,446, issued Jun. 21, 1977. The prior art also discloses techniques by which coatings of electrostatically charged particles can be developed progressively upon workpiece surfaces during movement thereof relative to a fluidized bed, as in Goodridge U.S. Pat. Nos. 3,828,729 and 3,914,461, issued respectively Aug. 13, 1974 and Oct. 21, 1975, and Westervelt et al U.S. Pat. No. 4,011,832, issued Mar. 15, 1977; non-electrostatic techniques, carried out similarly, are described in Goodridge U.S. Pat. Nos. 3,937,179 and 4,053,661, issued respectively on Feb. 10, 1976 and Oct. 11, 1977.
Efforts have been made in the past to utilize fluidized bed techniques, of both electrostatic and nonelectrostatic character, for developing powder coatings upon the inside surfaces of objects. Patents disclosing such concepts include Davis U.S. Pat. No. 3,004,861, issued Oct. 17, 1961, Barford et al U.S. Pat. No. 3,248,253, issued Apr. 26, 1966 (see FIG. 10), and Major et al United Kingdom Specification No. 925,021, published May 1, 1963. The Davis patent, in FIG. 2, shows apparatus for coating the inner surface of a tubular conduit utilizing a cup-like container, and container having a vertical wall that terminates in an outwardly flared lip and that closely approaches the surface to be coated. Powdered coating material is fluidized upon a porous plate disposed deeply within the container, and additional material may be supplied through a funnel member that is connected to a tube, which may extend either downwardly into the container or upwardly through the bottom thereof. In applying the coating material the container and conduit are moved relative to one another, and the patentee discloses that the thickness of the coating layer can be regulated by the rate of relative movement. Although a seal may be provided in the region of the flared lip of the container, it is deemed to be nonessential, because the amount of powder which would otherwise be lost is considered to be negligible.
The Major et al specification describes a method and apparatus for applying a coating of powered silica to the inside surface of an incandescent lamp envelope; in some cases the particulate material can be charged electrically by blowing it through a zone of ionization. The apparatus comprises a long glass tube, at the bottom of which is a diffusing pad covered by a layer of glass balls and, in turn, a reservoir located directly beneath the vessel being coated; particles that are too large to be sustained by the upwardly moving gas stream will be returned to the reservoir, and the delivery tube may be moved vertically within the object during the coating operation.
Certain fluidized bed units described in the art employ vacuum systems for exhausting fumes and recovering undeposited powder. Exemplary disclosures are set forth in Facer et al U.S. Pat. No. 3,560,239, issued Feb. 2, 1971, Huteaux U.S. Pat. No. 3,799,112, issued Mar. 6, 1974, and Walling et al U.S. Pat. No. 4,073,265, issued Feb. 14, 1978. A powder handling system, adapted for use with fluidized bed coating equipment, is described in Carlson et al U.S. Pat. No. 4,123,175, issued Oct. 31, 1978.
Despite the activity in the art exemplified by the foregoing, a need exists for means by which surfaces of workpieces can be coated quickly and efficiently with a particulate material from an electrostatic fluidized bed, which affords means for replenishing, in a optimal manner, the supply of particulate material in the fluidization chamber. Accordingly, it is the broad object of the present invention to provide a novel apparatus and method having such features and advantages.
Another object of the invention is to provide such an apparatus and method which are especially adapted for use in coating workpieces with a particulate material having abrasive properties.
Other objects of the invention are to provide such an apparatus which is relatively economical to build, and convenient to use.